1. Field of the Invention
The present invention relates to a rare earth metal-based permanent magnet having a corrosion-resistant film, and to a method for producing the same.
2. Description of the Related Art
Rare earth metal-based permanent magnets, for instance, R—Fe—B based permanent magnets wherein R is a rare earth metal, represented by a Nd—Fe—B based permanent magnet, or R—Fe—N based permanent magnets represented by a Sm—Fe—N based permanent magnet, etc., and particularly R—Fe—B based permanent magnets, are employed today in various fields because they utilize inexpensive materials abundant in resources, and possess superior magnetic properties.
However, since a rare earth metal-based permanent magnet contains a highly reactive rare earth metal, i.e., R, they are apt to be oxidized and corroded in the atmosphere, and in case they are used without applying any surface treatment, corrosion tends to proceed from the surface in the presence of small water as well as acidic or alkaline substances to generate rust. This leads to the degradation and the fluctuation in magnetic properties. Moreover, in case such a rusty magnet is embedded in a magnetic circuit and a like device, there is fear of scattering rust as to contaminate peripheral components.
In the light of the aforementioned circumstances, there is proposed a method of forming a corrosion-resistant film on the surface of the rare earth metal-based permanent magnet, and as a method for forming the corrosion-resistant film on the surface, there is proposed a method of forming a resin film by means of the application of resin, a method of forming a metal-plated film by means of wet plating, vapor phase plating, etc., or a method of forming a chemical conversion film such as a phosphate film or a chromate film, which are put into practice.
However, since there is formed a mixed phase consisting of a Nd2Fe14B phase having a noble oxidation-reduction potential as the principal phase and a Nd-rich phase having an oxidation-reduction potential lower than that of the principal phase as the grain boundary phase in the vicinity of the surface of a rare earth metal-based permanent magnet, for instance, a Nd—Fe—B based permanent magnet, it is known that electrochemical corrosion occurs based on potential difference depending on the potential differing from phase to phase.
If a corrosion-resistant film as described above is formed on the surface of the magnet, the corrosion based on potential difference can be suppressed as a result. However, the films above do not suppress the corrosion itself based on the difference in corrosion potential, but they are based on the concept of, so to say, sealing the corrosion depending on the corrosion potential by coating the entire surface of the magnet with a uniform film. Accordingly, since a film from several to several tens of micrometer in thickness is necessary to seal the corrosion depending on potential difference, a limit is automatically set in implementing a film with a high dimensional precision (i.e., in realizing a film as thin as possible, or in imparting high corrosion resistance while reducing thickness of the thin film). Furthermore, since complicated process steps are generally necessary in forming a resin film or a metal-plated film, these processes are not always advantageous in view of process cost. In case of forming a chromate film, moreover, it requires use of an ecologically unfavorable hexavalent chromium, which leads not only to a complicated waste treatment, but also to a fear of causing influence upon the human body on handling the magnet containing hexavalent chromium in a trace quantity.